Postal services, and in particular, the US Postal Service, charge for delivery of mail pieces by their weight, among other criteria. In general, the heavier the mail piece, the more is charged. While this may be a problem with individual private persons, for political reasons the US Postal Service (and others) does not generally target individual mail pieces that may be slightly overweight. Bulk mailers, however, who may mail thousands of pieces at a time, sometimes intermix heavier mail pieces with lighter ones and put the postage appropriate for the lighter pieces on every piece. This can occur, for example, when a bank mails out its customer statements. Most statements contain a few sheets and easily fit under the one ounce cut-off, but some of them contain many sheets and are overweight. Very often the banks do not put proper postage on the heavier pieces.
Audits, sometimes run by manually weighing suspect mail pieces, indicate that the US Postal Service loses many millions of dollars each year because of this practice. Because current methods of weighing mail pieces are either too slow for existing sorting machines or require individual or a set of mail pieces to be weighed manually, postal services have not devised methods for ensuring proper postage on each mail piece. It is important to remember that postage applies to each mail piece, not to the average weight within a set of mail pieces. As a result, a set of mail pieces whose average weight is under the limit may still contain many mail pieces that are individually too heavy and require greater postage.